It has been common practice to generate microwave energy for cooking purposes by a magnetron which is coupled to the cooking cavity by a waveguide either with or without the use of an "antenna" element projecting from the waveguide into the cavity. The energy is generated at an allocated frequency, for example, 915 or 2450 megahertz, and sets up standing waves within the cavity in modes which depends upon the physical dimensions and proportions of the cavity. In a cavity of the usual rectangular shape the modes are relatively simple and well defined, resulting in concentrations of energy at spaced anti-nodal points producing "hot spots" within the foodstuff so that portions of the foodstuff may become overdone or even burned, while adjacent portions are underdone.
In an effort to equalize the energy field it has been common practice to employ a rotating "stirrer" within the cavity having blades which cyclically vary the modes so that the hot spots are continually shifted within the foodstuff to more evenly distribute the heating effect.
However stirring devices are not completely effective and merely alleviate rather than cure the problem of hot spots. Also, stirring devices take up valuable space within or adjacent to the cavity. Efforts have been made to utilize a stirring device within the waveguide itself as a space conservation measure but, again, the effectiveness is limited. While it is possible to change the modes on a cyclical basis by modulating the frequencies of the waves, this introduces circuit complications and is an expensive solution.
In an effort to improve upon mode stirring it has been proposed to rotate an antenna extending from a waveguide into the oven cavity. By way of example, reference is made to Long U.S. Pat. No. 2,961,520 in which a spindle mounted axially within a hollow conductor is provided with a cross member extending sharply at right angles thereto to provide an antenna of unbalanced "T" configuration. A rotated antenna in the form of a dipole is also shown in Fritz U.S. Pat. No. 3,189,722. Such efforts have failed to produce equalized energy distribution, however, and have required resort to added elements such as glass plates, polarizing guides, or the like.
In analyzing devices of this kind it is helpful to consider two components of directly radiated wave energy. The first may be referred to as the "inertial" or unmodified wave which feeds directly into the cavity from the waveguide or hollow conductor. The other directly radiated component, which is superimposed upon the inertial wave, is the "guided" wave which is radiated by the portion of the antenna which is in the cavity and which differs in directivity from the inertial wave. In the absence of an auxiliary stirring device, it will be understood that the mode pattern of the inertial wave remains fixed and hence capable of producing hot spots, and it is only the guided component which is, by rotation of the antenna, continually shifted. Thus it is important to be able to control the directivity of the guided wave as well as to establish an energy ratio between the guided wave and the inertial wave.
However, analysis shows that rotated antennas of "L" or "T" shape, exemplified by the Long patent, produce a relatively low proportion of guided wave energy. It would appear that where the "crossbar" portion of the antenna forms a sharp rectangular joint with the spindle of shank portion, the crossbar simply acts as a discontinuity, impeding the feeding of inertial energy to the cavity and transmitting only a small amount of the total energy to the crossbar for radiation therefrom. Because of the relatively small amount of energy which is in guided form in the Long patent, and thus affected by the rotation, the main component of energy is in stationary, relatively simple modes highly affected by the dimensions and proportions of the cavity in which the antenna is used.
Our studies have shown that there is a still further factor which affects cooking efficiency and that is the extent to which the foodstuff is acted upon by direct radiation, that is, energy proceeding directly from the waveguide or antenna, as compared to the indirect radiation resulting from reflections within the cavity or reflections caused when blocking or altering the inertial wave. Increasing the amount of direct radiation reaching the foodstuff with controlled energy distribution not only improves the quality of cooking but also increases cooking efficiency. Analysis again shows that rotating antennas of sharply bent "L" or "T" configuration, being low in guided energy, are also low in the proportion of the energy reaching the foodstuff via a direct path, except when the foodstuffs are located centrally and in close proximity to the inertial wave.